You probably have a local "Mystery Spot" in your area, and it undoubtedly features a gravity hill — that is, a place where either water or solid objects seem to move uphill against the force of gravity. But do these uncanny spots really defy the laws of physics?

Most publicized gravity hills require a few bucks to get into, but in the San Francisco Bay Area you can see one for free. In Golden Gate Park, as you go down John F Kennedy Drive — just past the Shoreline Highway that crosses the park and just before you get to Lloyd Lake, which sports some fairly ill-advised fake classical ruins — you'll see a stream running just along the north side of the road. Keep moving along, and you will notice, just before the stream empties into the lake, it goes uphill. I have walked this way again and again for years, and I have never been able to convince myself that the stream isn't flowing up.

Many people all around the globe have found themselves happening upon physics dilemmas similar to this. Often they're on sloped roads in their cars, and when the car goes into neutral, they find themselves rolling up the slope of the hill. Occasionally, they drop something round, and watch it skitter up a hill.

The phenomenon has sparked a lot of ghost stories, from spectral children to Civil War soldiers trying to roll cannons into position. Since the illusion is most noticed on roads near hills, those with a scientific bent nickname it the Magnetic Hill. People think that the hill behind has to have some magnetic property that is influencing their cars. What's really influencing their cars is good, old-fashioned glitches in human perception.

Remember the joke where a grumpy old-timer tells kids that, in his day, they had to walk to school through the snow, uphill, both ways? That's not just a joke. People do tend to see inclines as being tilted up more than they seem as tilted down. Researchers at the University of Padova and the University of Pavia found that it's relatively easy to make people see something as uphill, and hard to make them see it as downhill.

They created a fake landscape out of angled boards. When the middle board was angled slightly downhill, and the boards around it were angled steeply downhill, most people they asked saw the middle board as going uphill. However, when they reversed the experiment, and showed people a slightly uphill stretch between two steeply uphill boards, it was seen as level, and not downhill. When the experimenters went on to test a horizontal board between two downhill stretches it was seen as uphill. When they tested a downhill segment between two uphill boards, it looked level. In other words, there wasn't anything they could do to make that middle section look downhill to anyone.

And these were just boards. Gravity hills make use of the entire landscape. First, most gravity hills are in places where a straight horizon is obscured in every direction. There isn't any horizon line to help people get their bearings. Usually the hill in question is a road cut into two uphill segments, but there are hills that are surrounded by downhill slopes. Generally, there are also trees and no buildings in the area. Buildings tend to stay perpendicular to the ground. Most of the time, trees do as well. When it comes to a choice between light and perpendicularity, though, the trees choose light every time. When they lean so that they're perpendicular to a downhill slope, it looks straight.

When everything, the contrasted slope of the landscape, the angle of the trees, and the inability to check anything against a horizon line, comes together on a slightly downhill slope, it will look uphill to whatever hapless chump happens to be wandering along it. Whenever someone checks with a level, or a GPS system the fun is over. Downhill is downhill. This illusion, though, is one that generally doesn't fade when you know the trick. So, if you have any mystery spots in your area, do let us know where they are, so we can roll up them.